The present invention relates to a control device for the pneumatic product tensioning and transport circuit in circular knitting machines for stockings and the like.
In circular machines for stockings, pneumatic circuits have been employed for some time to provide the tensioning of the stocking during the processing and to provide the transport thereof with its expulsion at the end of the processing.
These pneumatic circuits are composed of an aspiration chamber placed below the needle-bearing cylinder and connected through a conveyance conduit to an expulsion bell which in turn is connected to an aspirator. By operating the aspirator, a depression occurs in the aspiration chamber, which obtains the tensioning of the stocking during its forming and which allows, at the end of the processing, its transport up to the expulsion bell.
In the case of very long products, the disadvantage occurred of the twisting of the stocking during its formation and, for this reason, a device commonly known as anti-twist has been fitted in the aspiration chamber and the pneumatic circuit has been modified.
More in detail, a drilled pipe has been introduced into the aspiration chamber, which pipe rotates with the needlebearing cylinder, within which the stocking descends during its formation. By connecting the aspirator with the aspiration chamber as well as with the expulsion bell, and by placing a switching valve on the connecting conduits, according to the operation of the valve a first phase is obtained in which the aspirator is connected directly to the aspiration chamber so as to tension the stocking, which is held against the internal walls of the drilled pipe, and a second phase in which the aspirator is connected to the aspiration chamber through the transport conduit so as to suck the stocking at the end of the processing, transporting it out of the machine.
Generally, the switching from the first to second phase is obtained by means of cam profiles carried by a machine programing cylinder which, at the end of the processing of the stocking, by means of a system of leverages, acts on the cutoff element of the switching valve, varying the connection of the aspirator and which, after the start of a new processing, reestablishes the previous connection.
These known kinds of devices, though they achieve the tensioning of the stocking during the processing and provide its expulsion from the machine when processing is complete, have some disadvantages.
Among these, one of the main disadvantages is due to the fact that, between the programing cylinder and the switching valve, there is a series of mechanical transmissions which make the entire device very cumbersome and complicate its assembly, its tuning and its maintenance.
Another disadvantage is due to the fact that, every time the type of thread or of processing is changed, it is necessary to calibrate the valve manually so as to achieve during the tensioning phase a correct choking of the flow to increase or decrease the tensioning of the stocking during processing.